This invention relates to fluid-dispensing apparatus and methods for using the same, and more particularly to dispensing small volumes of fluid.
The ability to deliver small volumes of fluids accurately is important in a variety of industries. For example, a variety of different fabrication operations in the semiconductor industry utilize sub-microliter control of fluid dispensing. Such uses may require accurate, repeatable and rapid dispensing of precise amounts of fluids. If these requirements are not met, it may adversely impact the yield of the fabrication process.
One example of sub-microliter dispensing of fluids is in the fabrication of semiconductor light-emitting devices. It is known to provide semiconductor light-emitting device type light sources in packages that may provide protection, color selection, focusing and the like for light emitted by the light-emitting device. For example, the light-emitting device may be a light-emitting diode (“LED”). As shown in the example of FIG. 1, a power LED package 100 generally includes a substrate member 102 on which a light-emitting device 103 is mounted. The light-emitting device 103 may, for example, include an LED chip/submount assembly 103b mounted to the substrate member 102 and an LED 103a positioned on the LED chip/submount assembly 103b. The substrate member 102 may include traces or metal leads for connecting the package 100 to external circuitry. The substrate 102 may also act as a heatsink to conduct heat away from the LED 103 during operation.
A reflector, such as the reflector cup 104, may be mounted on the substrate 102 and surround the light-emitting device 103. The reflector cup 104 illustrated in FIG. 1 includes an angled or sloped lower sidewall 106 for reflecting light generated by the LED 103 upwardly and away from the LED package 100. The illustrated reflector cup 104 also includes upwardly extending walls 105 that may act as a channel for holding a lens in the LED package 100 and a horizontal shoulder portion 108.
As illustrated in FIG. 1, after the light-emitting device 103 is mounted on the substrate 102, a microliter quantity of an encapsulant material 107, such as liquid silicone gel, is dispensed into an interior reflective cavity 109 of the reflector cup 104. The interior reflective cavity 109 illustrated in FIG. 1 has a bottom surface defined by the substrate 102 to provide a closed cavity capable of retaining a liquid encapsulant material 107 therein. As further shown in FIG. 1, when the encapsulant material 107 is dispensed into the cavity 109, it may wick up the interior side of the sidewall 105 of the reflector cup 104, forming the illustrated concave meniscus.
In dispensing the encapsulant material 107, a bead of the material is typically formed on a dispensing needle and then contacted to surfaces of the reflective cavity 109 and the light-emitting device 103 therein. When the needle is withdrawn, the surface tension between the encapsulant material 107 and surfaces within the reflective cavity 109 and gravity cause the encapsulant material 107 to tear-off from the dispensing needle and remain in the reflective cavity 109.
While this surface tension controlled dispensing of the encapsulant material 107 may be very accurate under uniform conditions, a variety of factors may adversely impact the accuracy of the process and the amount of fluid dispensed. For example, different surfaces within the reflective cavity 109 may have different surface tension characteristics based on coatings on the surfaces, shape characteristics of the surfaces and variations in where the encapsulant material 107 is initially placed in the reflective cavity 109. In addition, variations in the characteristics of the encapsulant material 107 may also affect the amount of fluid dispensed. For example, the encapsulant material 107 is typically subject to varying viscosity and stringiness characteristics over time (due, for example, to partial curing) or across different temperature conditions. Stringiness characteristics, such as varying tail properties, may change with variations in temperature, humidity or the like or may change over time. Thus, the tear-off point and the volume of fluid dispensed may vary.
Other approaches to sub-microliter control of dispensing of fluids include the use of metering pumps particularly designed for accurate dispensing of small volumes of fluid. In addition, specially designed small volume dispensing nozzles (needles) are known that may be used with such precision pump systems.